Clint Eastwood is great in any role he fills. It's honestly more a matter of opinion than fact as to what his best genre is, as he's done excellent work in both settings. Who could forget The Outlaw Josey Wales, or Pale Rider? On the other hand, who hasn't asked some punk if they've felt lucky? Who hasn't wanted to blow a man head CLEAN OFF after watching Dirty Harry?
I don't think it's possible to place Clint Eastwood into a single best genre, because doing so is a disservice to his acting range.